Nyayasutra 2.15

2.15

tatra saṃśayādīnāṃ pṛthagvacanam anarthakam --- saṃśayādayo yathāsambhavaṃ pramāṇeṣu prameyeṣu cāntarbhavanto na vyatiricyanta iti? satyam etat, imās tu catasro vidyāḥ pṛthakprasthānāḥ prāṇabhṛtām anugrahāyopadiśyante, yāsāṃ caturthīyam ānvīkṣikī nyāyavidyā/

On this point, is the separate mention of doubt and the rest useless, since doubt and the others are included within the means of valid cognition and the objects of valid cognition according to their nature, and are not distinct? That is true; however, these four disciplines, having distinct methodologies, are taught for the benefit of living beings, of which this critical inquiry, the science of reasoning, is the fourth.